Does High CE Explain Martensite Formation?

[tc7]

I understand that high carbon alloys (~ 0.3- 0.4% and greater) require post weld stress relief and thermal treatment because of the likely formation of hard martensite that needs to be tempered and that the ability to form martensite is related to the carbon content.

But what about other chrome molys that have a low carbon content (< 0.15%) but have a high CE value due to high chrome and Moly, i.e., 2.25% Cr & 1%Mo? Is martensite formation still possible with this alloy even though it has low C?

If so, suppose I maintain a high preheat temperature, say 400 deg F? will this be sufficient to slow the cooling rate through the HAZ and prevent formation of martensite?

 

[metengr]

tc7

But what about other chrome molys that have a low carbon content (< 0.15%) but have a high CE value due to high chrome and Moly, i.e., 2.25% Cr & 1%Mo? Is martensite formation still possible with this alloy even though it has low C?

Yes. There is a good article in the recent AWS Welding Journal that explains the correlation with carbon equivalent equation, prehat requirements and hardness (related to formation of martensite). Also here is a web site that shows predicted hardness related to chemical composition;

http://homepage3.nifty.com/yurioka/exp.html

 

[arunmrao]

low carbon martensite is possible . The alloying elements will definitely push the nose of C curve towards right thus enabling martensitic transformation. However, the carbon content in the martensite will be of a lower order.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[Mr168]

Metengr: Do you happen to know what issue of the AWS WJ that article was in? Mine gets shipped to a different address, and I'd like to have it sent to me for a look.

Thanks

 

[metengr]

Mr168;

November, 2008

"Estimating Welding Preheat Requirements for Unknown Grades of Carbon and Low Alloy Steel"

by Hinton and Wiswesser

 

[strider6]

Hiher CE means higher likelihood to form martensite and hard phase.As mentioned in by metengr in the article of WJ is stated that the addition of alloy element like NI and Cr lower the maternsite start and finish temperature.

Also there are good source of information on the web like this from Steel Matter:

"An increase in Mo content shifts the CCT and TTT curves to the right and also separates the ferrite + pearlite region from the bainite region making the attainment of a bainitic structure more controllable."

http://www.matter.org.uk/steelmatter/metallurgy/7_1_2.html

and this

Martensite and Martensitic Phase Transformations

http://www.msm.cam.ac.uk/phase-trans/2002/martensite.html

hope this help

S


Corrosion Prevention & Corrosion Control

 

[tc7]

Metengr -
Thanks for the WJ reference, it will be a week before my copy arrives.


I would interpret the "move to the right" will cause slower cooling rates to more readliy result in martensite formation rather than pearlite formation - would this be true?

Arunmrao - what is the significance of "carbon content in the martensite will be of a lower order" ? will this simply mean that whatever martensite is formed that it will be in a less hardened state? or will it mean that overall less martensite will be formed and leave a greater percentage of austenite?

 

[metengr]

I would interpret the "move to the right" will cause slower cooling rates to more readily result in martensite formation rather than pearlite formation - would this be true?

Correct.

Lower order I believe would refer to lower hardness. Besides a potent hardener for martensite, carbon is an austenite stabilizer, so increased carbon would tend to increase retained austenite upon quenching.

 

[CoryPad]

Yes, lower order means lower hardness. A low carbon martensite is going to have lower hardness than a high carbon martensite. The Cr and Mo alloying can increase the hardness some, but its main effect is to increase the hardening depth.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arunmrao]

metengr and corypad have rightly pointed out that a low carbon martensite will result in lower hardness. This is because martensite will be stressed lower due to carbon (lesser) entrapment. alloying elements will not have a significant effect on hardness but more on hardenability.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[tc7]

So in conclusion, if the martensite is less hard when an alloy is lower in carbon such as in a 2.25Cr-1Mo (F22 alloy with only 0.15% C max), is a post weld stress relief or thermal treatment less important than say for a 4130 or 4340 alloy of 0.3% or 0.4% carbon?

 

[arunmrao]

Not necessarily true. A martensitic transformation poses all proceesing problems like quench cracks, grinding cracks distortion etc. A tempering treatment is essential to eliminate the problems.

I produce mandrel casting having C 0.15%, Ni 4.5%, Cr 1.2% and Mo 1.2%. In my ignorance, I removed the feeder heads by oxy acetylene cutting,and then did snag grinding of the surface. These were then annealed and during machining, beautiful cracks appeared on the gas cut surface .

I learnt my lesson of treating low carbon martensite with respect that day .

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)